Monoclonal antibody, which exhibits high selectivity to a specific antigen, has attracted considerable attention as an effective drug in recent years. In particular, there are great expectations to develop an antibody drug that targets cancer cells. In order to apply a monoclonal antibody as an effective drug to the treatment of humans, administration of a human antibody having a small amount of heterogenetic antigen is most ideal from the viewpoint of prevention of immunological rejection. Thus, a large number of chimeric antibodies and humanized antibodies have been developed.
In general, a chimeric or humanized antibody used in the treatment of humans is produced by immunizing a mouse or the like with an antigen several times, then fusing a cell collected from the spleen or lymph node with a myeloma cell to form a hybridoma, and then applying a recombinant technique to a mouse IgG antibody produced by the hybridoma.
However, it takes a long period of time to treat individual animals and to select hybridomas that produce antibodies with high affinity. Moreover, it is necessary to confirm the activity of the antibody obtained by the recombinant technique. Thus, this method requires a long period of time to produce an antibody of interest, and further, the effects of the antibody cannot be confirmed until the antibody is administered to a human.
Furthermore, when an immunogen exhibits whole body toxicity, it is difficult to immunize an individual with it. Further, when a protein antigen which is highly conserved among animal species is used as an antigen, it is difficult to produce an antibody due to immunological tolerance.
On the other hand, it has been known that a B cell exhibiting high affinity for an antigen is selected in the germinal center. However, such a selection mechanism has not yet been sufficiently elucidated. If it were possible to allow a B cell exhibiting high affinity for a specific antigen to artificially grow and to concentrate it, a monoclonal antibody exhibiting high affinity for a specific antigen could be produced in a shorter time.
As a method for allowing a B cell to grow, there has been known a method of culturing a B cell in the presence of a CD40 ligand (CD40L) and cytokine such as interleukin (IL)-4 (for example, National Publication of International Patent Application No. 9-512441 and J Exp Med. 1992 Vol. 176(6): pp. 1543-1550).
The germinal center is a histological structure formed as a result of the phenomenon that an antigen-non-contacted naive B cells is allowed to come into contact with an antigen and is thereby allowed to grow. It has been known that class switch and somatic hypermutation occurs in the B cells in the germinal center. For example, Abstract of the Annual Meeting of the Japanese Society for Immunology, 2007, Vol. 37, p. 259, 3-F-W41-16-O/P discloses that, when splenic B cells are cultured together with IL-4 and an anti-μHc antibody in the presence of fibroblasts in which BAFF and CD40L have been co-expressed, almost all of the B cells exhibit a germinal center-like phenotype, and approximately a half of the cells are class-switched to the IgG1, and that thereafter, if IL-4 is changed to IL-21, the number of IgE-positive cells increases.